1. Field of the Invention
This invention relates to variable resistance devices such as a potentiometer. Specifically, this invention relates to an electrical resistance element of a variable resistance device and the method of making such a resistance element whereby the resistance function of the element is designed to provide a selected non-linear resistance. More specifically, this invention relates to a potentiometer having a thick film electrical resistance element with at least one section of the element having a selected series of alternating, generally transverse cuts which may be made by a laser.
2. Description of the Prior Art
Resistance elements are known which provide non-linear electrical resistance functions. Also, it is known to use these elements as part of a variable resistance device.
There are generally two methods of making non-linear resistance elements which are pertinent in regard to the present invention. The first method requires using two or more materials to construct the resistance path of the element. Two materials are either placed sequentially along the resistance path or one material is placed alongside the resistance path in electrical contact therewith. Both methods require multiple printing of the resistance material onto the non-conducting substrate of the element. Multiple printing and firing of the resistance material in the manufacturing process is expensive and time consuming since each material must be printed and heat treated separately thereby doubling or tripling the handling of each element. In addition, the rejection or fracture rate increases with each handling, printing and firing of the element. The second method requires physical tailoring or cutting away of the conductive path, that is, making a longitudinal cut in the material to give a selected resistance function. This method requires the predetermination of the shape and path to be cut and elaborate controls to guide and regulate the longitudinal cut. The tailored shape is not easily adjusted to compensate for irregularities and differences in the resistance material from element to element.
Resistance elements of a variety of constructions are known for use in fixed resistance devices. The most pertinent known element in regard to the present invention includes a thick film resistance element having transverse cuts made in the resistance material by a laser beam. The cuts increase the current path length through the material thereby increasing the total resistance of the element. The increase in path length is a function of the length, width and location of the cuts. This type of resistance element is used to make fixed resistance devices and to the best of our knowledge has not heretofore been used in variable resistance devices.